Numerous vaccine formulations which include attenuated pathogens or subunit protein antigens, have been developed. Conventional vaccine compositions often include immunological adjuvants to enhance cell-mediated and humoral immune responses. For example, depot adjuvants are frequently used which adsorb and/or precipitate administered antigens and which can retain the antigen at the injection site. Typical depot adjuvants include aluminum compounds and water-in-oil emulsions. However, depot adjuvants, although increasing antigenicity, often provoke severe persistent local reactions, such as granulomas, abscesses and scarring, when injected subcutaneously or intramuscularly. Other adjuvants, such as lipopolysacharrides, can elicit pyrogenic responses upon injection and/or Reiter's symptoms (influenza-like symptoms, generalized joint discomfort and sometimes anterior uveitis, arthritis and urethritis). Saponins, such as Quillaja saponaria, have also been used as immunological adjuvants in vaccine compositions against a variety of diseases.
More particularly, Complete Freund's adjuvant (CFA) is a powerful immunostimulatory agent that has been successfully used with many antigens on an experimental basis. CFA includes three components: a mineral oil, an emulsifying agent, and killed mycobacteria, such as Mycobacterium tuberculosis. Although effective as an adjuvant, CFA causes severe side effects primarily due to the presence of the mycobacterial component, including pain, abscess formation and fever. CFA, therefore, is not used in human and veterinary vaccines.
Incomplete Freund's adjuvant (IFA) is similar to CFA but does not include the bacterial component. IFA, while not approved for use in the United States, has been used elsewhere in human vaccines for influenza and polio and in veterinary vaccines for rabies, canine distemper and foot-and-mouth disease. However, evidence indicates that both the oil and emulsifier used in IFA can cause tumors in mice.
Despite the presence of such adjuvants, conventional vaccines often fail to provide adequate protection against the targeted pathogen. In this regard, there is growing evidence that vaccination against intracellular pathogens, such as a number of viruses, should target both the cellular and humoral arms of the immune system. More particularly, cytotoxic T-lymphocytes (CTLs) play an important role in cell-mediated immune defense against intracellular pathogens such as viruses and tumor-specific antigens produced by malignant cells. CTLs mediate cytotoxicity of virally infected cells by recognizing viral determinants in conjunction with class I MHC molecules displayed by the infected cells. Cytoplasmic expression of proteins is a prerequisite for class I MHC processing and presentation of antigenic peptides to CTLs. However, immunization with killed or attenuated viruses often fails to produce the CTLs necessary to curb intracellular infection. Furthermore, conventional vaccination techniques against viruses displaying marked genetic heterogeneity and/or rapid mutation rates that facilitate selection of immune escape variants, such as HIV or influenza, are problematic. Accordingly, alternative techniques for vaccination have been developed.
Particulate carriers with adsorbed or entrapped antigens have been used in an attempt to circumvent these problems and in attempts to elicit adequate immune responses. Such carriers present multiple copies of a selected antigen to the immune system and promote trapping and retention of antigens in local lymph nodes. The particles can be phagocytosed by macrophages and can enhance antigen presentation through cytokine release. Examples of particulate carriers include those derived from polymethyl methacrylate polymers, as well as polymer particles derived from poly(lactides) and poly(lactide-co-glycolides), known as PLG. While offering significant advantages over other more toxic systems, antigen-containing PLG particles suffer from some drawbacks. For example, large scale production and manufacturing of particulate carriers may be problematic due to the high cost of the polymers used in the manufacture the particulate carriers.
Liposomes have also been employed in an effort to overcome these problems. Liposomes are microscopic vesicles formed from lipid constituents such as phospholipids which are used to entrap pharmaceutical agents. Although the use of liposomes as a drug delivery system alleviates some of the problems described above, liposomes exhibit poor stability during storage and use, and large scale production and manufacturing of liposomes is problematic.
International Publication No. WO 98/50071 describes the use of viral-like particles (VLPs) as adjuvants to enhance immune responses of antigens administered with the VLPs. St. Clair et al. describe the use of protein crystals to enhance humoral and cellular responses. (St. Clair, N. et al, Applied Biol. Sci., 96:9469-9474, 1999).
Despite the above described adjuvant and antigen-presentation systems, there is a continued need for effective, safe and cost-efficient vaccines with improved purity, stability and immunogenicity.